Composites have been used in an increasing number of applications to replace metal where lower weight, corrosion resistance and greater fatigue resistance has been required or desirable. Composite spring elements are well known. Various patent disclosures reveal either composite structures alone or in combination with other materials, such as rubber, especially in automotive applications. As an example, U.S. Pat. No. 2,559,105 discloses a composite metal and rubber leaf spring, while U.S. Pat. No. 3,142,598 discloses a different type of leaf spring using resin impregnated fiberglass. Epoxy/glass fiber springs have been introduced which are interchangeable with conventional steel leaf springs in automobiles.
However, the introduction of these composites has not been without problems. Most current composite spring applications are limited by the manufacture employed and the inherent anisotropic nature of resin impregnated glass fiber. While pretensioning the glass fibers in the epoxy resin matrix by the techniques of pulforming or pultrusion is old and well known, a continuing limitation in the manufacture of composite beams such as springs has been the constant or uniformly thick cross-section required and the essentially uniform fiber distribution in that cross section. The ability to successfully shape composite springs without creating the potential for failure due to buckling because of exposed glass fibers that breed surface micro-cracks in the epoxy retaining matrix has heretofore not been successfully accomplished. Further, the ability to tailor the strength of the final composite structure by varying the positioning of the glass fibers relative to the cross section of the structural beam has not heretofore been successfully accomplished. These problems are solved in the design of the present invention that employs a constant stress beam with a gradient fiber distribution in the plane of deflection.